The objective of this project is to learn more about how molecular oxygen, a normally unreactive, but vital substance, reacts with biological systems. Complexes of molecular oxygen with reduced flavins and flavoproteins, and with microsomal cytochrome P-450 oxygenases will be characterized by anaerobic and rapid reaction technique. These complexes are believed to be functional forms of activated oxygen which take part in hydroxylations and oxidations vital to metabolic regulation and detoxification processes. An unknown but important value to be determined in this research is the electrochemical potential for the univalent reduction of oxygen to superoxide.